Univention Corporate Server 4.1 erratum 351 (bugfix)

This update addresses the following issue(s):
* Added the command "univention-app dev-set" to support development tools.
* The ini attribute "License" is now passed to the frontend.
* Adjust code so that other projects may extend the App Center lib.
* The documentation has been extended.
* For developers, reverting a local App Center does remove the App Center
  directories completely.
* Hiding dockerized versions of installed Apps did not work when upgrading
  from UCS 4.0.
* When upgrading from a Non-Docker version to a Docker version, the old
  version was removed even the installation process of the new version was
  not successful.